Ultrasonic transducers have been available for quite some time and are useful for interrogating solids, liquids and gasses. One particular use for ultrasonic transducers has been in the area of medical imaging. Ultrasonic transducers are typically formed of piezoelectric elements. The elements typically are made of material such as lead zirconate titanate (abbreviated as PZT), with a plurality of elements being arranged to form a transducer assembly. The transducer assembly is then further assembled into a housing possibly including control electronics, in the form of electronic circuit boards, the combination of which forms an ultrasonic probe. This ultrasonic probe, which may include acoustic matching layers between the surface of the piezoelectric transducer element or elements and the probe body, may then be used to send and receive ultrasonic signals through body tissue.
One limitation of piezoelectric devices is that the acoustic impedance of the piezoelectric material is approximately 30-35 MRayls (one MRayl being 1*10.sup.6 kg/m.sup.2 s), while the acoustic impedance of the human body is approximately 1.5 MRayls. Because of this large impedance mismatch acoustic matching layers are needed to match the piezoelectric impedance to the body impedance. Acoustic matching layers work using a 1/4 wave resonance principle and are therefore narrow band devices, their presence thus reducing the available bandwidth of the piezoelectric transducer.
In order to achieve maximum resolution, it is desirable to operate at the highest possible frequency and the highest possible bandwidth.
In order to address the shortcomings of transducers made from piezoelectric materials, a micro-machined ultrasonic transducer (MUT), which is described in U.S. Pat. No. 5,619,476 to Haller, et al., has been developed. Micro-machined ultrasonic transducers address the shortcomings of piezoelectric transducers by, among other attributes, being fabricated using semiconductor fabrication techniques on a silicon substrate. The MUT's are formed using known semiconductor manufacturing techniques resulting in a capacitive non-linear ultrasonic transducer that comprises, in essence, a flexible membrane supported around its edges over a silicon substrate. By applying electrical contact material to the membrane, or a portion of the membrane, and to the silicon substrate and then by applying appropriate voltage signals to the contacts, the MUT may be energized such that an appropriate ultrasonic wave is produced. Similarly, the membrane of the MUT may be used to detect ultrasonic signals by capturing reflected ultrasonic energy and transforming that energy into movement of the membrane, which then generates a receive signal. When imaging the human body, the membrane of the MUT moves freely with the imaging medium, thus eliminating the need for acoustic matching layers. Therefore, transducer bandwidth is greatly improved.
A drawback associated with MUTs, however, is that because of the manner in which transducer cells are arranged on a substrate, significant portions of the surface area of the MUT element is devoted to support structure for the MUT membranes. Unfortunately, the support structure is acoustically inactive, thus degrading the overall sensitivity of the MUT element
Therefore it would be desirable to have a number of applications in which a MUT may be employed and which may improve the performance of a MUT.